The symbiotic plant structures called mycorrhizae are formed when certain nutritionally exacting fungi become integrally associated with plant roots. Rather than acting pathogenically, these fungi enhance the growth of the infected plant by the increased uptake of nutrients and water and by the production of growth factors essential for higher plants. Mycorrhizal fungi, in return, utilize plant carbohydrates as a source of energy for metabolic processes.
Ectotropic mycorrhizal (ectomycorrhizal) fungi are important components of forest ecosystems in which the fungi infect the roots of pines and other trees in this symbiotic manner to enhance tree survival and growth, even in distressed soils. An example of the beneficial aspects of mycorrhizal fungi has been described by Marx (The Ohio Journal of Science, v. 75, no. 6, pgs. 288-297, 1975) in his attempts to increase pine seedling survival on spoiled lands. Marx reported a 35 percent increase in survival of pine seedlings specifically inoculated with a mycorrhizal fungus, Pisolithus tinctorius, compared to uninoculated seedlings. Unfortunately, the procedures described by Marx for cultivating mycorrhizal fungi on solid media are very time consuming and do not lend themselves to a large-scale production.
Other literature references to growth of mycorrhizal fungi can be found in: Ruehle and Marx, "Fiber, Food, Fuel and Fungal Symbionts", Science, v. 206, pgs. 419-422 (Oct. 26, 1979); Santoro and Casida, "Improved Method for Obtaining Vegetative Growth of Mycorrhizal and other Slow Growing Fungi", J. Bacteriology, v. 78, pgs. 449-450 (1959); van Eybergen and Scheffers, "Growth of the Mycelium of Boletus edulis on Agar Media and in Submerged Liquid Cultures", Antonie van Leeuwenhock, v. 38, pgs. 448-450 (1972). These publications generally disclose liquid cultures of species of Boletus and Cenococcum, which are non-sporulating, ectomycorrizal fungi, but they do not disclose the mass production of the specific sporulating organisms of the present invention.
The present invention is directed toward the mass cultivation of sporulating, ectomycorrhizal fungi in liquid culture. Such a procedure permits the continual, large-scale production of mycorrhizal fungi, for example, in a chemostat-like device used for continual liquid culture of bacteria. Such large-scale production is suitable for direct inoculation of tree roots or inoculation of soil prior to planting. In subcultural practices these techniques could lead to improvements in reclamation of strip-mined lands. In agricultural practices, inoculation with mycorrhizal fungi may lead to increased crop production with a decreased dependence on expensive, chemical fertilizers.